I just got back from costa rica last night! All the rivers are pretty dry right now, especially in guanacoste which is where your wife wants to go. you might have a hard time kayaking if you are only travelling with your wife, i would recomend sticking to surfing, depending on how liong you are there.
I stayed at a place in Playa Grande called Las Tortugas which was really chill and had a beach with great waves. You can rent boards there from 10-15 dollars per day. A guy named sergio teaches lessons for about 20$ but he's a freak, i'd almost reccomend figuring it out on your own. Grande is within walking distance to tamarindo but far enough away from the huge crwods of tourons. I also really liked Playa Avellanas because it has good waves for you, more shade for the wifey, and less crowds. its really windy in guanacoste right now, which is good for the waaves but bad for getting sand in your eyes.
If you want more jungle with your beach go down to the osa peninsula whic is really remote and nice or over to the carribean side which will be much wetter. Saw lotsa animals on the osa- parrots, monkeys, crocodiles, etc. and they have the longest break in north amercia somewhere down there. other than that there are great mountains just east of san josefor rivers, but you're pretty far from the beach and san jose kinda sucks. check out the la paz waterfall gardens to drool over 85-120 foot waterfalls. Some look runnable with a good flow! anyways, have fun bro, pura vida!